The main function of the skin consists in providing protection between the hostile earthly world and the interior of the organism. This defensive function is made possible by the two following main functions:
(i) an epidermal barrier function. It is located on the exterior portion (stratum corneum) of the most superficial layer of the skin (the epidermis). The structure of the corneous layer can be compared to a wall of bricks, wherein the bricks represent the dead cells (cornocytes) and the mortar, an intercellular matrix organized into a lipid lamellar structure. This barrier protects the organism from chemical, microbiological and physical aggressions and regulates the exchanges in water between the organism and the environment;
(ii) the cutaneous immune system (CIS). The CIS is able to induce an inflammatory response against exterior aggressions. The CIS can be divided into 2 portions, an innate portion and an adaptive portion:                The innate immune system corresponds to the first-line mechanism, which is non-specific against the microbial invasion. In the case where the physical barrier is broken, the innate immune response is activated in order to eliminate the aggressors. The innate system uses pre-existing receptors which can bind to molecules and/or molecular patterns commonly found on many microbes. However, this innate immune system remains unable to develop a memory of aggressions and therefore effective protection with regards to reinfection;        The adaptive immune system consists of a delayed and specific immune response. It is capable of recognizing more specifically exterior and antigen aggressions, destroying them and retaining in memory the aggressions incurred.        
Keratinocytes are cells that constitute 90% of the epidermis and appendages, i.e. the nails, hair, fur, feathers, and scales. They play an important role in the local immune function. They indeed produce interleukins and cytokines aiming to preserve the homeostasis i.e. the balance of the barrier.
The antimicrobial peptides (AMPs) of the innate immune system, such as cathelicidins or beta-defensins, which are produced by the skin, are stored in lamellar bodies and are then delivered to the corneous layer.
These antimicrobial peptides AMPs are small peptides having between about 12 and 50 amino acids. They are considered as one of the key elements of the innate immune system, servant for the defense of multicellular organisms during microbial infections. They have properties of direct inhibition against microbial pathogens. They attach and fit into the microbial phospholipid bilayer, inducing a inducing a rupture of the microbial membrane then a lysis.
There are a large number of AMPs. The main known AMPs are beta-defensins (BD) and cathelicidins (Cath). Beta-defensins and cathelicidins have been identified in the skin (epidermis) of a large number of mammals, in particular humans, the dog, the cat and the horse. These AMPs are synthesized by leukocytes and epithelial mucosal tissue. However, they are mainly produced by keratinocytes. They are expressed or induced by bacterial components (lipopolysaccharides) or by pro-inflammatory mediators (by way of example: IL-6 for Interleukin-6, IL-8, interferon-gamma, TNF-alpha).
The beta-defensins (BD) have antimicrobial activity and play an immunomodulatory role (by chemotactic activity). In animals and in particular in dogs, existing data on the expression of beta-defensins and in particular cBD103 is contradictory. As such, the mRNA expression of cBD103 in atopic dogs would be lower than the mRNA expression of cBD103 in healthy dogs according to studies presented by Van Damme et al., Mol Immuno 2009. This mRNA expression between atopic dogs and healthy dogs would be similar according to studies presented by Leonard et al., J Innate Immunol 2012. Finally, according to the studies of Santoro et al., Vet Derm 2013 the mRNA expression of cBD103 in atopic dogs would be increased with respect to healthy dogs.
Likewise, according to Santoro et al., Vet Derm 2013, the mRNA expression of cBD103 in skin lesions (lesional sites) would be increased with respect to non-lesional sites. On the other hand, according to studies presented by Leonard et al., J Innate Immunol 2012, the mRNA expression of cBD103 would substantially similar between lesional and non-lesional sites.
The cathelicidins (Cath) are also powerful antimicrobial agents. They have an intrinsic ability to kill Gram-negative and Gram-positive bacteria, fungi and viruses. They also promote a host response by triggering the recruitment of immunocompetent cells and release of cytokines.
It has been shown that an alteration of the expression of AMPs in subjects with an alteration of the skin barrier (for example during skin lesions or during dermatitis, such as canine or feline atopic dermatitis, Malassezia dermatitus, during pyoderma, chronic ear infections, or fungal or bacterial infections) may predispose the host to secondary infections. The expression levels of beta-defensins and cathelicidin of atopic dogs are modified relative to their expression levels in healthy dogs.
The alterations of the skin barrier can be induced by the pathology in a primary manner or secondarily by pruritus. Atopic dermatitis in particular is a chronic pruritic skin disease that appears in numerous species. Animals with atopic dermatitis have a genetic predisposition to develop allergic reactions with respect to environmental antigens, such as pollen or mold. This results in excessive production of reaginic antibodies (IgE). An estimated 10% of dogs suffer from atopy. Skin lesions generated by this disease are greatly aggravated by licking, scratching, and bacterial infections. Atopic dogs furthermore exhibit high rates of Staphylococcus pseudintermedius and Malassezia pachydermatis. 